1. Field of the Invention
The present invention relates to an improved zinc phosphating method. More particularly, it relates to an improved zinc phosphating method for enhancing the corrosion resistance of a metal substrate at the edge portion.
2. Statement of Related Art
In recent years, the corrosion resistance of metal substrates has been remarkably improved by the development of zinc phosphating and the introduction of cationic electrodeposition. In a coating film formed on the surface of a metal substrate, however, blisters are frequently produced on the edge portion and develop within a short period of time. Prevention of such blister production has not been successful up to the present time.
As the result of the microscopic observation of the surface of a metal substrate which was zinc phosphated and then electrodeposition coated, it has been found that at the outer edge position, a zinc phosphate coating is present but the electrodeposition coating is almost nonexistent due to flowing on baking. In general, a zinc phosphate coating can contribute to enhancement of the corrosion resistance but does not have satisfactory corrosion resistance by itself, because of its microporosity.